I want to take my wife and 18 year old to NZ in June/July for 10 days. We thought about 5 days on each island. They are worried it's going to be too cold. Are most of the lodges open that time of year? Should we stay in one spot on each islands and make daily sojourns from the lodge? Any trip recommendations would be welcome.
I suggest that you visit the official website of the Government of New Zealand .... www.newzealand.com for travel advice. You will be arriving at the 'start' of winter, in a very southernly land. Although there is loads to see and do, you may want to bring your skiis if you are thinking about travel to the interior of the southern island. The best skiing in the world in June and July, and that may appeal to you.
The best way to see New Zealand is to travel in a self drive car, and go from place to place -- its a big piece of real estate. If 'too cold' is something that concerns you, stick to the North Island (its probably warmer, but who knows, its an island country!!). Summer in New Zealand is now, December thru March, and the place is fantastic. But there are good reasons to visit New Zealand any time of the year. It all depends on what you want. You may want to stay two nights in any one place, and travel or relax and then move on. There is a really good itinery elsewhere on the 'destinations' blog for Australia / NZ.
You can rent a self drive (car / camper), and use it on the North Island, and when / if you want to go to the South Island, the rental car company will meet you at the ferry, take back your car, you hop on the ferry, and pick up a new car on arrival at the Southern Island.
I have been in Wellington and Auckland in June/ July. Not a winter by wind swept north american standards (not a Canadian winter to be sure), but a winter that would be consistent with the Easternseaboard of the US. A North Carolina winter, or equivalent. I carried an umbrella alot, and used it. And was happy that I brought a sweater or two, but was not 'miserable' by any means.
If the classic NZ summer is what you are counting on.... that's a January trip, or Feb. That said, NZ is a great holiday destination. You have to decide what it is that you want to see, whales, vineyard festivals, sun and surf, or some of the best skiing in the southern hemisphere, and pick the time that suits your wishes.
Check the NZ tourism web above. It will give you an indication of what festivals happen when, and the weather, and a full range of accommodation options among other things. That should help you on your way. I enjoy visiting NZ any time of the year. Its just one of those gems that few people have opportunity to visit. Good on you for thinking about NZ as a travel option.